The present invention relates in general to the detection of corrosion on a steel surface underlying a non-magnetic coating thereon.
The detection of corrosion underneath a protective coating on the surface of steel has been traditionally achieved by electrochemical techniques involving the use of electrolytes which render detection cumbersome and time consuming. Non-destructive evaluation techniques have also been utilized for detection of defects, involving ultrasound, eddy current, radiography or thermography reflecting changes in base metal caused by the defects. Such non-destructive evaluation techniques detect changes in mass density, elastic stiffness or conductivity of an electrical or thermal type in physically local environments of metal oxide mixtures or metal voids created by corrosion. Since typical steels have expectedly wide variations in properties associated with such local environments, it would be difficult to distinguish between such variations and those resulting from corrosion hidden underneath a coating on the steel. Also some of the foregoing existing techniques are sensitive to material thickness and geometrical effects unrelated to corrosion so as to render corrosion detection unreliable.
The detection of hidden corrosion in aluminum alloys involving use of nuclear magnetic resonance, is disclosed for example in U.S. Pat. No. 5,905,376 issued May 18, 1999. The corrosion detection technique, as disclosed in such patent, is not however applicable to steels. It is therefore an important object of the present invention to provide a reliable technique for detection of corrosion in the form of magnetic oxidation products hidden underneath a protective coating covering a targeted surface of steel.
In accordance with a corrosion detection method of the present invention, microwave energy of an appropriate frequency is absorbed in a body of steel by transmission through a protective coating on its targeted surface when a static DC magnetic field of less than 0.5 Telsa is externally applied thereto. Such absorbed microwave energy reflected from the steel body is measured through a portable sensor to provide microwave measurement data that is compared by coordination with data on variations in the strength of the magnetic field before and after corrosion of the targeted steel surface for reliable and readily available corrosion detection.